As shown in JP-2000-104600A, in an engine control system, air-fuel ratio or rich/lean of exhaust gas is detected by an exhaust gas sensor (air-fuel ratio sensor or oxygen sensor), and air-fuel ratio (fuel injection quantity) is feedback corrected based on the detected value in such a manner that the air-fuel ratio becomes consistent with a target air-fuel ratio. This feedback correction amount is learned, and its learning value is stored in a backup RAM which is a rewritable non-volatile memory. The backup RAM holds the data by using of in-vehicle battery even while the engine is stopped. Based on the learning value, the air-fuel control is performed.
When the in-vehicle battery is removed from the vehicle, the backup power for the backup RAM is interrupted, so that learning data stored in the memory are erased, which is referred to as a battery-clear. After the battery-clear, it is necessary to perform a learning of the feedback correction amount from the first (initial value). During a period until the learning is completed, an accuracy of the air-fuel ratio control is deteriorated. Hence, it is desirable to reduce the learning period after the battery-clear.
JP-61-28739A shows that an updating speed (learning speed) of the learning value, after the battery-clear, is accelerated until a specified period has elapsed from a starting of engine.
An update amount of the learning value per one learning is increased to accelerate the learning speed.
FIG. 2 is a time chart showing a conventional system. In this system, when a variation width of the air-fuel ratio feedback correction amount is within a stable determination value, the correction value is learned. After the battery-clear, a learning speed is increased by moderating a learning condition until a specified time period for completing the learning has passed
After the specified time period has elapsed and the learning has been completed, the learning speed is varied to ordinary low speed in order to avoid an erroneous learning. After that, if an abnormality arises in the air-fuel ratio control system (for example, intake air system, fuel supply system, and the like), the air-fuel ratio feedback correction amount may rapidly change as shown in FIG. 2. FIG. 2 shows a behavior of the system in which a pipe of fuel vapor treatment system, which is connected to an intake pipe, is displaced.
After the learning is completed, the learning speed of the correction amount is maintained at low speed even if an abnormality arises in the air-fuel ratio control system and the correction value is rapidly changed. Hence, a long time period is required to complete the learning of the correction amount. That is, a long time period is required to converge the learning value to a stable value after a rapid change of the correction amount.